Analog-to-digital conversion of a radiocommunication signal

ABSTRACT

A method for analog/digital conversion of a radiocommunication signal using an analog analog/digital converter preceded by variable gain amplifier, wherein the frequency band of the converted signal contains at least one useful channel, comprises steps for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter. However, the power in the useful channel is substantially equal to a mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

This application is a 371 of PCT/FRO1/02290 filed on Jul. 13, 2001.

FIELD OF THE INVENTION

The present invention relates to a method of analog/digital conversionof a radiocommunication signal, with the aid of an analog/digitalconverter preceded by a variable-gain amplifier. The converted signalhas a determined frequency band containing at least one useful channel,in general from among several different channels. A useful channel isthe frequency band of minimum width making it possible to recover auseful signal.

The invention finds applications in radiofrequency receivers, such asmay be found for example in the fixed equipment (base stations) or themobile equipment (portable terminals) of radiocommunication systems. Insuch an application, the signal considered is for example aradiofrequency signal such as a phase-modulated and/oramplitude-modulated carrier, or a signal resulting from thetransposition of such a signal to an intermediate frequency or tobaseband. The useful signal contains information coding voice and/ordata and/or signaling information.

BACKGROUND OF THE INVENTION

In state of the art receivers, the frequency band of the convertedsignal corresponds to the useful channel. Accordingly, theradiofrequency signal received on a reception antenna is filtered, ingeneral after transposition to an intermediate frequency or to baseband,by means of a filter disposed in the reception chain upstream of theanalog/digital converter. In this way, the latter converts only theenergy of the signal in the useful channel. When the receiver is amultichannel receiver, channel selection means, comprising a selectivefilter, make it possible to select the useful channel from among aplurality of different channels. In a manner known per se, avariable-gain amplifier can precede the analog/digital converter in thereception chain, so as to tailor the power level in the useful channelto the input dynamic swing in power of the converter. This is achievedby virtue of appropriate automatic gain control means.

Attempts are currently being made to dispose the analog/digitalconverter nearer to the reception antenna, and in particular upstream ofthe channel selection means. Hence, the signal converted by theanalog/digital converter has a determined frequency band containing atleast one useful channel, in general from among several differentchannels. The width of the frequency band of the converted signal, orconverted band, is then greater than that of the useful channel. Thisconverted band is determined by the (analog) filters disposed upstreamof the converter. The channel selection means, disposed downstream ofthe analog/digital converter in the reception channel, then comprise amixer and one or more digital filters for selecting the useful channel,before digital demodulation and decoding.

However, the power of the radiofrequency signal received on thereception antenna varies over time. These variations may be due to theappearance or the disappearance of obstacles between the transmitter andthe receiver, to the appearance or the disappearance of other signals inthe frequency band occupied by the signal, or to “fading” when there isa relative motion of the receiver with respect to one of thetransmitters. One speaks of propagation of dynamic type in a channelwhen there is a relative motion of the receiver with respect to thecorresponding transmitter, and of propagation of static type in theconverse case. Fading is considerable when the Doppler frequency off₀×v/c is considerable, where fo is the central frequency of thechannel, v is the relative speed of the receiver with respect to thetransmitter and c is the speed of light. It is noted that when a signalis situated in a “fading hole” its power may become very small. Thedecrease in power of the signal in a “fading hole” is of short duration.In fact, the shorter the duration of the “fading hole”, the smaller thepower of the signal in the “fading hole”.

Moreover, the input dynamic swing in power of the analog/digitalconverter is limited above by an upper limit value beyond which theconverter is saturated, and below by a lower limit value beneath whichthe signal can no longer be distinguished from the noise introduced bythe converter. One speaks of saturation value to designate said upperlimit value, and of noise floor to designate said lower limit value. Byconvention, the desired power levels and the mean power values indicatedsubsequently in this document may be expressed in decibels (dB) withrespect to the noise floor of the converter.

Furthermore, any radiocommunication system complies with specificationswhich determine the sensitivity and the rejection of the system withregard to the useful channel, as a function of the type of propagationin this channel. The sensitivity of the system corresponds to theminimum power level of the signal in the useful channel (signal-to-noiseratio), at which the system must still operate. The rejection of thesystem corresponds to the maximum power level which must be tolerated bythe system inside the converted band, in the channels neighboring theuseful channel. The sensitivity in the static case is less than thesensitivity in the dynamic case, and the rejection in the static case isgreater than the rejection in the dynamic case. In one example, thedynamic sensitivity is equal to 15 dB above the noise floor of theconverter, and the dynamic rejection is equal to 45 dB. Moreover, thestatic sensitivity is equal to 7 dB above the noise floor of theconverter and the static rejection is equal to 70 dB. The input dynamicswing in power of the converter which is necessary is therefore equal to60 dB in the dynamic case, to 77 dB in the static case, and hence to 85dB to cover both the static case and the dynamic case if the usefulsignal level is fixed at the same level in both cases. At a rate of 6 dBper digit, a converter operating on 15 bits at output is thereforenecessary.

SUMMARY OF THE INVENTION

An object of the present invention is to propose a method and a devicewhich make it possible to use a converter operating on a smaller numberof bits at output, and which is therefore less expensive.

Accordingly, the invention proposes a method of analog/digitalconversion of a radiocommunication signal with the aid of ananalog/digital converter preceded by a variable-gain amplifier, thefrequency band of the converted signal containing at least one usefulchannel, consisting in controlling the value of the gain of theamplifier in such a way that the power in the frequency band of theconverted signal is less than a limit saturation value of theanalog/digital converter and that the power in the useful channel issubstantially equal to a mean power level having a first predeterminedvalue in the case of static propagation in the useful channel or asecond predetermined value, different from said first predeterminedvalue, in the case of dynamic propagation in the useful channel.

By distinguishing the static case from the dynamic case, it is possibleto decrease the input dynamic swing in power of the converter. Thus,returning to the values of the example above, the values of the gain ofthe amplifier can be controlled in such a way that the power in theuseful channel is substantially equal to 7 dB above the noise floor ofthe converter in the static case, and to 15 dB above the noise floor ofthe converter in the dynamic case. In this way, the input dynamic swingin power of the converter which is necessary to cover both cases isequal to 77 dB. A converter operating on 13 bits at output is thereforesufficient, and is much less expensive than a converter operating on 15bits at output.

The invention also proposes a device for analog/digital conversion of aradiocommunication signal whose frequency band contains at least oneuseful channel, comprising an analog/digital converter preceded by avariable-gain amplifier, and means for controlling the value of the gainof the amplifier in such a way that the power in the frequency band ofthe converted signal is less than a limit saturation value of theanalog/digital converter and that the power in the useful channel issubstantially equal to a desired mean power level in the useful channelhaving a first predetermined value in the case of static propagation inthe useful channel or a second predetermined value, different from saidfirst predetermined value, in the case of dynamic propagation in theuseful channel.

The invention further proposes a radiofrequency radiocommunicationreceiver incorporating such a device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the simplified diagram of a radiofrequency radiocommunicationreceiver incorporating a device according to the invention;

FIG. 2 is the diagram of a unit for measuring the power in the convertedband or in the useful channel;

FIG. 3 is a curve illustrating the profile over time of the power of thesignal in a useful channel;

FIG. 4 is a flow chart of the steps of the method according to theinvention;

FIG. 5 and FIG. 6 are flow charts detailing substeps respectively of astep of determining the type of propagation in the useful channel and astep of determining a “fading hole” of the signal in the useful channel.

DETAILED DESCRIPTION

Represented in FIG. 1 is the diagram of a radiofrequency receiverincorporating a device according to the invention. The receivercomprises a reception antenna 10 linked to the input of a radiofrequencyamplifier 11 which outputs a radiofrequency signal RF. This is forexample a phase-modulated and/or amplitude-modulated signal. In oneexample, the spectrum of the RF signal lies in the 380-500 MHz band. Itcomprises several distinct channels which are, for example, all of thesame width.

The RF signal is carried to a first input of a first mixer 12 a. Asecond input of the mixer 12 a receives a signal at a frequency f_(LO1)less than the frequency of the RF signal, delivered by a first localoscillator 13 a. In one example, the frequency f_(LO1) is equal to 154MHz. The mixer 12 a outputs a signal IF1 which corresponds to the RFsignal transposed to the intermediate frequency f_(LO1). The signal IF1is filtered by means of a first bandpass filter 14 a. The output of thefilter 14 a is linked to a first input of a second mixer 12 b. A secondinput of the mixer 12 b receives a signal at a frequency f_(LO2) lessthan the frequency f_(LO1) of the signal IF1, delivered by a secondlocal oscillator 13 b. In one example, the frequency f_(LO2) is equal to500 KHz. The mixer 12 a outputs a signal IF2 which corresponds to the RFsignal transposed to the intermediate frequency f_(LO2). The signal IF2is filtered by means of a second bandpass filter 14 b. The signal IF2thus filtered corresponds to the radiocommunication signal S accordingto the invention.

The device according to the invention comprises a variable-gainamplifier 15 whose input is linked to the output of the filter 14 b soas to receive the signal S, and whose output delivers the amplifiedsignal S. The device further comprises an analog/digital converter 16whose input is linked to the output of the amplifier 15 so as to receivethe amplified signal S and whose output delivers discrete instantaneousvalues or samples Sn of this signal. The sampling frequency f_(e) of theconverter 16 is for example equal to 2 MHz, complying with the Shannoncondition.

Specifically, by reason of the passband of the bandpass filters 14 a and14 b, the frequency band of the signal S, called the converted band, isaround 150 KHz wide. Stated otherwise, it is assumed that the rejection,in terms of power, is big enough outside of this band for it to bepossible to neglect the spectral components of the signal S outside ofthis band. Moreover it is centered on the frequency 500 KHz. In oneexample, a useful channel lying in the converted band is considered tobe centered on the frequency 450 KHz. The converted band comprisingseveral channels such as the useful channel, the bandwidth of thischannel is less than that of the converted band.

The device comprises a unit 18 for measuring the power in the convertedband, whose input receives the samples Sn, possibly but not necessarilyvia a digital filter 17 whose passband corresponds to the width of theconverted band. The device also comprises a unit 20 for measuring thepower in the useful channel, whose input receives the samples Sn via achannel selection module 19. The module 19 comprises means of digitaltransposition, for transposing the useful channel to baseband. In theexample, these means comprise a digital mixer making it possible totranspose the converted band in such a way that it is centered on 50KHz. The useful channel is then centered on 0 Hz. The module 19 alsocomprises digital means of low-pass filtering making it possible toisolate the components of the useful signal. The samples S′n output bythe module 19 are also transmitted to the downstream part 23 of theradiofrequency receiver, which here is represented overall by a box.This downstream part 23 comprises in particular the means ofdemodulation and of decoding of the useful signal, which make itpossible to extract the data transmitted in the useful signal.

The units 18 and 20 produce respectively values P_(B) of the power inthe converted band and values P_(C) of the power in the useful channel,which are provided at the input of a management unit 21 of the device.Preferably, the device comprises, for example, in the power measuringunits 18 and 20, means for compensating for the difference of delay inthe transmission of the values P_(B) and P_(C) which is due to thedifference of the paths taken. This makes it possible to deliver at theinput of the unit 21 values of the power in the converted band and inthe useful channel referring to identical samples Sn. The managementunit 21 comprises means for implementing a method according to theinvention, and outputs a control signal for an automatic gain controlmodule 22. The module 22 produces an analog signal which is carried to again control input of the variable-gain amplifier 15, so as to controlthe gain of this amplifier in the manner which will now be described.

The gain of the amplifier is preferably controlled as a function ofvalues of the mean power in the converted band and/or in the usefulchannel, in order for the device not to be too sensitive to the slightfluctuations in the power of the signal received. Specifically, takingaccount of values of the instantaneous power would give rise to changesin the gain of the amplifier which could prove to be inopportune in thesense that they might destabilize the reception chain. The values of themean power are calculated over a determined time window. The larger thistime window, the less sensitive is the device to the fluctuations in thepower of the signal received. From another point of view, themeasurement of the mean power of the signal is then available only afterthe expiry of this time window. This lag may be penalizing in certaincases, in particular on switching on the receiver. Specifically, itcauses a delay in the adjusting of the reception chain which takes placein a receiver initialization phase.

This is why, according to an advantageous characteristic of theinvention, the units 18 and 20 each produce N series of values, denotedP₁n to P_(N)n in what follows, of the mean power respectively in theconverted band (aforesaid values P_(B)) and in the useful channel(aforesaid values P_(C)), where N is an integer, the values of theseseries being calculated over increasing time windows of respectivewidth. In one example, N is equal to 5.

Represented in FIG. 2 is the simplified diagram of a power measurementunit such as the aforesaid unit 18 or the unit 20. In what follows, theterms “input signal” used with reference to the units 18 and 20designate the signal corresponding respectively to the series of samplesSn and S′n provided as input to the units 18 and 20, that is the samplesof the signal in the converted band and in the useful channelrespectively. For the unit 18, the series of values P₁n to P_(N)ncorrespond to the values of the power in the converted band which areindicated overall by the reference P_(B) in FIG. 1, while for the unit20, the values P₁n to P_(N)n correspond to the values of the power inthe useful channel which are indicated overall by the reference P_(C) inFIG. 1. The description of a power measurement unit which follows withreference to FIG. 2 relates to the example of the unit 18 receiving theseries of samples Sn as input signal. Given that the unit 20 isidentical to the unit 18, this description also holds, with thenecessary alterations to the notation, for the unit 20 receiving theseries of samples S′n as input signal.

The power measurement unit of FIG. 2 optionally comprises a subsamplingmodule 101 which carries out a subsampling of the samples of the inputsignal, at a subsampling frequency f_(se) which is a submultiple of thesampling frequency f_(e). In one example, f_(se)=f_(e)/125, so that onesample Sn out of 125 is transmitted by the module 101. The values of thesamples Sn are for example coded on p bits, where p is an integer.

The unit further comprises a module 102 for calculating instantaneouspower receiving as input the series of values Sn. This module has thefunction of producing a series of values Pn of the instantaneous powerof the input signal, from the series of values Sn. The values Sn beingwritable in the form of an imaginary number Sn=S_(I)n+i. S_(Q)n, whereS_(I)n and S_(Q)n are real numbers and where i²=−1, the values Pn areobtained successively from the successive values Sn by performing foreach the calculation Pn=S² _(I)n+S² _(Q)n. The values Pn are thereforecoded on 2p bits.

The unit further comprises, according to the invention, N mean powercalculation modules arranged in cascade, where N is an integer. Each ofthese modules, referenced 103 ₁ to 103 _(N) in FIG. 1, allows thecontinuous production of the series of values P₁n to P_(N)n respectivelyof the mean power of the input signal, calculated over increasingrespective time windows directly or indirectly from the values Pn of theseries of values of the instantaneous power of the input signal. Themodules 103 ₁ to 103 _(N) are in what follows called modules forcalculating mean power of level 1 to N respectively. These aresynchronous modules.

The module 103 ₁ for calculating mean power of level 1 comprises amemory register 104 ₁, as well as a counter C₁ (not represented)counting up to N₁, where N₁ is an integer such that N₁≧2, (notrepresented) and means for resetting to zero the register 104 ₁ and thecounter C₁ (also not represented). It further comprises means ofaddition 105 ₁, a first input of which is coupled to the output of thecircuit 102 for calculating instantaneous power so as to receive thevalues Pn of the instantaneous power of the input signal, a second inputof which is coupled to an output of the register 104 ₁ so as to receivethe current value stored in this register, and the output of which iscoupled to the input of said register 104 ₁. With each reception of anew value Pn, the means of addition 105 ₁ produce a value equal to thesum of said value Pn and of said current value stored in the register104 ₁ this sum value being then stored in the register 104 ₁ in place ofsaid current value. Stated otherwise, the hereinabove means of themodule 103 ₁ form an accumulator register. Such a register is of verysimple structure and requires little memory space, since the register104 ₁ must have a length enabling it to store the result of the additionof N1 words of 2p bits, that is equal to 2p+N1 only.

The module 103 ₁ for calculating the mean power of level 1 outputs aseries of values P₁n which are obtained successively for example byaveraging over N₁ successive values Pn of the instantaneous power of theinput signal. Preferably, this is an arithmetic mean, which is thesimplest to implement since it requires just one complex step ofdivision by N₁. Accordingly, the counter is incremented by one unit witheach reception of a new value Pn and corresponding updating of the valuestored in the register 104 ₁. When the counter reaches the value N₁ thevalue stored in the register 104 ₁ is divided by N1 to yield anarithmetic mean of the last N₁ successive values Pn of the instantaneouspower of the input signal which were received at the input of the module103 ₁. A value P₁n of the mean power of level 1 of the input signal isthus produced. Moreover, the value of the counter C₁ and the valuestored in the register 104 ₁ are reset to zero. Preferably, the integerN₁ is an integer power of 2, that is there exists a nonzero integer k₁such that N₁=2^(k) ¹ . This makes it possible to simplify the step ofdivision by N₁ since it is then sufficient to eliminate the N₁ leastsignificant bits of the value stored in the register 104 ₁ to producethe value P₁n.

Each circuit 103 _(j) for calculating the mean power of level j, where jis an index such that 2≦j≦N, produces a j-th series of values P_(j)n ofthe mean power of level j of the input signal from N_(j) values of thej−1-th series of values P_(j−1)n of the mean power of level j−1 of theinput signal, where N_(j) is an integer such that N_(j)≧2. It isnecessary to distinguish between the last module 103 _(N) (for whichj=N) and the other modules 103 _(j) (for which 2≦j<N).

For the values of j such that 2≦j<N, the values of the j-th series ofvalues P_(j)n of the mean power of the input signal are obtainedsuccessively by averaging over successive N_(j)-tuples of successivevalues of the j−1-th series of values P_(j−1)n of the mean power oflevel j−1 (immediately lower level) of the input signal. Preferably,this is an arithmetic mean, which is the simplest to implement since itrequires few complex calculations.

Accordingly, each module 103 _(j) for calculating the mean power oflevel j can have the same structure as the module 103 ₁ for calculatingthe mean power of level 1 described hereinabove, with a counter C_(j)counting up to N_(j).

Nevertheless, in a preferred exemplary embodiment, each module 103 _(j)for calculating the mean power of level j comprises, in place of thememory register 104 ₁ of the module 103 ₁, a shift register 104 _(j) oflength N_(j), that is comprising N_(j) elementary registers in series,as well as a counter C_(j) (not represented) counting up to N_(j) andmeans (also not represented) for resetting to zero the counter C_(j) andoptionally the register 104 _(j). It further comprises means of addition105 _(j) with N_(j) inputs which are linked respectively to the outputsof the N_(j) elementary registers 104_(j) so as to receive the N_(j)values stored in the shift register 104 _(j). The input of each module103 _(j) is coupled to the output of the module 103 _(j−1) so as toreceive the values P_(j−1)n and its output is coupled to the input ofthe module 103 _(j+1) so as to send it the values P_(j)n. Each time avalue P_(j−1)n is input into the shift register 104 _(j), the counterC_(j) is incremented by one unit. When N_(j) values P_(j−1)n of the meanpower of level j−1 (immediately lower level) have been input into theshift register 104 _(j), that is when C_(j)=N_(j), these N_(j) valuesare added together in the adder 105 _(j). The sum obtained is thendivided by N_(j) to produce a value P_(j)n of the mean power of level jof the input signal. Moreover, the shift register 104 _(j) can beemptied of the values which it contains, by virtue of the abovementionedmeans for resetting to zero. Preferably, each integer N_(j) is aninteger power of 2, that is there exists an integer k_(j) such thatN_(j)=2^(k) ^(j) . This simplifies the step of division by N_(j), as hasbeen set forth previously.

The structure of the modules 103 _(j) for calculating mean power oflevel j for 2≦j<N thus enable them to keep in memory, in the shiftregister 104 _(j), the previous values of the mean power of level j−1.This history of the values of the mean power can thus be used at anyinstant, as will be described hereinbelow.

Let us now see the particular case of the last module 103 _(N). Thevalues of the last series of values P_(N)n of the mean power of level Nof the input signal are obtained successively by taking a slidingaverage over the successive K_(N)-tuples of the last N_(N) values of theN−1-th series of values P_(N−1)n of the mean power of level N−1(immediately lower level) of the input signal.

Accordingly, the module 103 _(N) for calculating the mean power of levelN can have the same structure as the module 103 ₁ for calculating themean power of level 1 described earlier, with a counter C_(N) countingup to N_(N), but which is not reset to zero after the calculation ofeach value P_(N)n.

Nevertheless, in a preferred exemplary embodiment, the module 103 _(N)for calculating the mean power of level N of the input signal comprisesa shift register 104 _(N) of length N_(N), that is comprising N_(N)elementary registers in series, and an adder 105 _(N) with N_(N) inputsfor receiving respectively the N_(N) values stored in the shift register104 _(N), where N_(N) is an integer. The input of the module 103 _(N) iscoupled to the output of the module 103 _(N−1) for calculating the meanpower of level N−1 (immediately lower level). With each input of a newvalue P_(N−1)n of the mean power of level N−1 of the input signal intothe shift register 104 _(N), the values which are stored therein areshifted so that the oldest value P_(N−1)n stored in the shift register104 _(N) is lost. A counter C_(N) (not represented) able to count up toN_(N) is incremented by one unit with each input of a new value P_(N−1)ninto the shift register 104 _(N). Moreover, a sum of the N_(N) valuesnewly stored in this register is calculated by virtue of the means ofaddition 105 _(N). As soon as the counter has reached the value N_(N)(C_(N)≧N_(N)), the sum thus obtained is divided by N_(N) to produce thevalue P_(N)n of the mean power of level N of the input signal, accordingto (preferably) an arithmetic mean. Preferably, the integer N_(N) is aninteger power of 2, that is there exists an integer k_(N) such thatN_(N)=2^(k)N, this simplifying the step of division by N_(N) as was setforth previously. This calculation produces a value P_(N)n of the meanpower of level N of the input signal. The counter C_(N) is not reset tozero after the calculation of each value P_(N)n.

The modules 104 ₁ to 104 _(N) for calculating the mean power of level 1to N respectively of the input signal are for example embodied in theform of hardware and/or software modules, for example in amicrocontroller, an ASIC circuit, a DSP circuit, an FPGA circuit, or thelike.

As will have been understood, the successive values Pn of theinstantaneous power of the input signal which are delivered by thecircuit 102 bring about the cascaded generation of the series of valuesP₁n to P_(N)n of the mean power of level 1 to N respectively of theinput signal. Thus, a value P₁n of the mean power of level 1 is a valueof the input signal mean power calculated over a time window of widthequal to N₁ times an elementary duration separating two successivevalues Pn of the instantaneous power of the input signal. Thiselementary duration is equal to $\frac{1}{{fe} \times {fse}}.$

Likewise, a value P₂n of the mean power of level 2 is a value of theinput signal mean power calculated over a time window of width equal toN₁×N₂ times this elementary duration. Expressed in a general manner,this signifies that a value P_(j)n of the mean power of level j of theinput signal is a value of the signal S mean power calculated over atime window of width equal to N₁×N₂× . . . ×N_(j−1)×N_(j) times theduration separating two consecutive values Pn of the instantaneous powerof the input signal. These time windows are therefore of respectiveincreasing widths.

Thus, the more the level j of the mean power of the input signalincreases, the more the small variations in the values of the inputsignal are masked in the value P_(j)n of this mean power of level j.Nevertheless, the smaller the level of this mean power, the more quicklyare the values P_(j)n of the mean power available after the unit hasbeen switched on. In one example, N is equal to five, N₁, N₄ and N₅ areequal to eight, and N₂ and N₃ are equal to two. Moreover, the values ofthe power in the converted band and/or the values of the power in theuseful channel which are taken into account in an initialization phaseare the values P₁n of mean power of level 1 which are calculated over atime window having a first determined width, while the values of thepower in the converted band and/or the values of the power in the usefulchannel which are taken into account in a holding phase are the valuesP₅n of mean power of level 5 which are calculated over a time windowhaving a second determined width, greater than said first determinedwidth. Of course, values of the mean power of different levels may betaken into account for the power in the converted band and for the powerin the useful channel. In one example, the initialization phase isconsidered to begin when the device is brought into service and toterminate as soon as a value P₅n of the mean power of level 5 in theuseful channel is available. However, the device is brought back to theinitialization phase and the memory registers and the counters Ci for ilying between 1 and N of the units 18 and 20 are reinitialized with eachmodification of the value of the gain of the amplifier 15. Moreover, andpreferably, the values of the power in the frequency band of theconverted signal and/or the values of the power in the useful channelwhich are taken into account are calculated on the basis of measurementsof the instantaneous power after a first determined lag has elapsedafter bringing into service or modifying a parameter of an analog partupstream of the analog/digital converter.

The values P_(B) of the power in the converted band and the values P_(C)of the power in the useful channel which are produced respectively inthe units 18 and 20, are by nature decimal values on a linear scale.They are for example expressed in watts (W) or in milliwatts (mW).Moreover, the mean power values are calculated from values in watts orin milliwatts. Nevertheless, it is advantageous to express them indecibel milliwatts (dBm), that is on a logarithmic scale. Specifically,the values of the gain of the amplifier 15 which can be controlled bythe gain control signal delivered by the management unit 21 aregenerally expressed in dB. Likewise, the saturation value Psat and thenoise floor Pmin of the converter 16 are generally expressed in dBm inthe specifications. Also, the mean power level P_(C)o desired in theuseful channel and the mean power level P_(B)o desired in the convertedband are expressed in dBm. As was stated earlier, all these values canbe expressed through a deviation in dB with respect to the value Pmin ofthe noise floor of the converter expressed in dBm. Likewise, variousmargins used in the comparison steps can be expressed in dB. It isindeed advantageous to deal with values expressed in dB, sinceoperations of multiplication or of division on values expressed linearlyare then performed by means of simpler operations of addition and ofsubtraction respectively.

This is why the values P_(B) of the power in the converted band and thevalues P_(C) of the power in the useful channel are converted intovalues in decibels by means of a predetermined conversion table storedfor this purpose in the units 18 and 20 respectively. Such a table cantake the form given by table I below. In the unit 20, there exists sucha table for each type of power measurement of a channel (samplingfrequency, channel filter used) lying in the frequency band of theradiocommunication signal S. In each of these tables, each columncorresponds to one of the predetermined values which can be taken by thegain of the amplifier 15. These gain values go from a minimum value Gminto a maximum value Gmax with a stepsize of for example 1 dB.Advantageously, the power values converted into decibels by means of thetable are then independent of the current value of the gain G of theamplifier 15.

Likewise each row of the table corresponds to a measured power valuegoing from a minimum value Pmin, which corresponds to the value of thenoise floor of the converter possibly increased by a margin, to amaximum value P_(B)max (for the power in the converted band) or P_(C)max(for the power in the useful channel), with a stepsize ΔP of for example0.5 dB. Each row of the table therefore corresponds to an index j suchthat the power value indicated in this row corresponds to Pmin+j.ΔP forj going from 0 to Np, where Np is an integer.

TABLE I Gain Index P_(B) Gmin . . . . . . . . . . . . . . . Gmax 0 PminX X X X X X X 1 Pmin + ΔP X X X X X X X 2 Pmin + 2.ΔP X X X X X X X . .X X X X X X X . . . . j Pmin + j.ΔP X X X X X X X . . X X X X X X X . .. . Np Pmin + NpΔP X X X X X X X

The conversion of any power value is carried out as follows. The valueto be converted, expressed in watts or in milliwatts, is compared withthe values from the column of the table corresponding to the currentvalue of the gain G of the amplifier 15, which values are expressed inthe same unit (W or mW). It is perhaps equal to one of these values orlies between two of these values contained in two adjacent rows of thetable. If it is less than Pmin or greater than Pmin+NpΔp, it is set toPmin or to Pmin+NpΔp, respectively. From this is then deduced the valueof the index j corresponding to the row of the table whose value, forthe relevant column, is closest to the value to be converted. The valueof this index is saved in memory and is used to compare the power valuewith other power values converted in the same way. It was seen that Pmincorresponds to the zero value of the index j. In one example, Psatcorresponds to the value Np of the index j. Stated otherwise,Psat=Pmin+Np.Δp. Index values (integers) are thus compared, instead ofcomparing values in watts or in milliwatts (decimal numbers). This issimpler. Moreover, the index values can be saved in place of thecorresponding values in watts or in milliwatts. This occupies less roomin memory.

In FIG. 3, the curve 50 represents an example of the profile versus timeof the power P_(C) in a determined useful channel. Horizontal linesrepresent an interval around a desired predetermined mean power levelP_(C)o which is regarded as satisfactory for the channel taking intoaccount the input dynamic swing in power of the analog/digital converter16. This interval is delimited by an acceptable maximum value P_(C)maxand by an acceptable minimum value P_(C)min.

In case of static propagation in this channel, that is when thecorresponding transmitter is fixed with respect to the receiver, thepower P_(C) hardly varies over time. Its slight fluctuations are dueonly to parasitic glitches in the channel. In case of dynamicpropagation in the channel, that is when the corresponding transmitteris moving with respect to the receiver, the power P_(C) varies somewhatmore, and it may pass below the level P_(C)min as indicated for exampleby the reference 52 in FIG. 3, or above the level P_(C)max.

According to the invention, the desired mean power level P_(C)o, andpossibly also the deviation between the acceptable minimum levelP_(C)min and/or acceptable maximum level P_(C)max on the one hand andP_(C)o on the other hand, depend on the type of propagation in theuseful channel. In one example, the values of P_(C)o, P_(C)min andP_(C)max are respectively equal to 4 dB, 7 dB and 10 dB above the noisefloor Pmin of the analog/digital converter 16 (P_(C)min=Pmin+4 dB;P_(C)o=Pmin+7 dB; P_(C)max=Pmin+10 dB) in the static case, andrespectively to 12 dB, 15 dB and 18 dB above Pmin (P_(C)min=Pmin+12 dB;P_(C)o=Pmin+15 dB; P_(C)max=Pmin+18 dB) in the dynamic case.

The type of propagation, static or dynamic, can be determined as afunction of the state of the receiver (when the latter comprises meansfor detecting that it is moving), or of data received from thetransmitter (when the latter comprises means for detecting and signalingthat it is moving). Nevertheless, the device according to the inventionpreferably comprises means for determining the type of propagation,static or dynamic, in the useful channel as a function of the history ofthe values of the power in the useful channel which are obtained in theabsence of saturation of the analog/digital converter as will beexplained in greater detail hereinbelow in conjunction with FIG. 5.

When the signal in the channel is in a “fading hole”, the power P_(C) inthis channel may suddenly become less than the minimum level P_(C)min,as indicated for example by the references 51 and 53 in FIG. 3. However,this sudden decrease in the power in the useful channel is of shortduration. Consequently, it may be preferable not to modify the gain ofthe variable-gain amplifier 15 (FIG. 1) on account of the powermeasurements in such a “fading hole”.

This is why the device comprises means for determining whether thesignal in the useful channel is in a “fading hole”, and for modifyingthe gain of the amplifier 15, as appropriate, only if the signal in theuseful channel is not in a “fading hole”. A “fading hole” is detected bythe abrupt variation of the successive values of the power P_(C) in thechannel, as will be explained in greater detail hereinbelow inconjunction with FIG. 6.

The manner of operation of the device, according to the method of theinvention, is described hereinbelow with reference to the flow chart ofFIG. 4.

When the receiver is switched on, the device operates according to aninitialization phase. Subsequently, it operates according to a holdingphase. The method of analog/digital conversion whose steps arerepresented on the flow chart of FIG. 4 and implemented both during theinitialization phase and during the holding phase. It will be now bedescribed in the case of the initialization phase. This description is,making the necessary alterations which will be pointed out, also validfor the implementation in the holding phase. It will therefore not berepeated for the latter, so as to avoid a redundancy.

The method begins with a step 31 consisting in assigning a predeterminedinitial value to the gain G of the amplifier 15, which normally makes itpossible to avoid the saturation of the analog/digital converter 16. Astep 32 then consists in comparing a value P_(B) of the power in theconverted band with the limit saturation value Psat. If P_(B) is notgreater than Psat minus a predetermined margin, we then go to a step 34.If on the contrary P_(B) is greater than Psat minus said margin, then,in a step 33, the gain G of the amplifier 15 is decreased in such a waythat the power P_(B) in the converted band is substantially equal to adesired value P_(B)o less than or equal to Psat minus said margin. Forthis purpose, two cases are distinguished. If P_(B) is less than orequal to P_(B)max, then the current value G of the gain of the amplifier15 is replaced by G+P_(B)o−P_(B). If on the contrary P_(B) is strictlygreater than P_(B)max, then the current value G of the gain of theamplifier 15 is replaced by G−ΔG, where ΔG constitutes a relatively highvariation in the gain relative to the gain stepsize of the conversiontable (which is 0.5 dB). For example ΔG equals 2 dB. After step 33 wereturn to the comparison step 32. In this way, possibly after severaliterations of step 33, the gain G of the amplifier 15 is such that thepower P_(B) in the converted band is at most substantially equal to thevalue P_(B)o of the mean power level desired in the converted band.

Step 34 consists in determining the type of propagation, static ordynamic, in the useful channel as a function of the history of thevalues P_(C) of the power in the useful channel which are obtained inthe absence of saturation of the converter 16. It will be detailedhereinbelow in conjunction with the flow chart of FIG. 5. It is followedby a step 35 of determining a possible “fading hole” in the usefulchannel. This step 35 will be detailed hereinbelow in conjunction withthe flow chart of FIG. 6.

Next, the method comprises a step 36 consisting in comparing a valueP_(C) of the power in the useful channel with the mean power levelP_(C)o desired in this channel.

More exactly, a check verifies whether the value P_(C) lies in theinterval around P_(c)o defined by the values P_(C)min and P_(C)max. IfP_(C) is not outside this interval, then the end 38 of the method hasbeen reached. Conversely, if P_(C) is outside this interval, then, in astep 37, the gain G of the amplifier 15 is modified so that the valueP_(C) is inside said interval. However, this modification of the valueof the gain G must not entail any risk of the saturation of theconverter 16. This is why a test for validating change of the gain isperformed so as to verify that the envisaged new value of the gain doesnot entail any risk of saturation of the converter 16. The new value ofthe gain which is envisaged to replace the current value G is forexample G+P_(C)o−P_(C). The test for validating the change of gainconsists in comparing the value P_(B) of the power in the converted bandwith the limit saturation value Psat of the converter 16. More exactly,if (P_(C)o−P_(C))+P_(B)≦Psat−margin, then the envisaged new value ofgain can be adopted since there is no risk of it causing the saturationof the converter 16. Otherwise, the new value of the gain must belimited to Psat−margin−P_(B). In this way, priority is given to theavoiding of the saturation of the converter 16 over the obtaining of thebest possible sensitivity in the useful channel. Stated otherwise thegain of the amplifier is controlled in such a way that the power in theuseful channel is substantially equal to the predetermined mean powerlevel P_(C)o. Thus, the gain of the amplifier is increased at most byonly a value such that the power in the converted band remains less thanthe limit saturation value Psat of the converter 16 minus saiddetermined margin.

After step 37 we are at the end 38 of the method. Nevertheless, steps 32to 37 may be repeated cyclically both during the initialization phaseand during the holding phase.

Represented in FIG. 5 is a flow chart showing substeps of step 34 ofdetermining the type of propagation in the useful channel.

During substep 341, the values P_(C)i of the instantaneous power of theuseful channel are produced and kept over a duration T. The value of Tdepends on a speed v of the mobile above which the mobile is regarded asbeing in the dynamic regime and below which the mobile is regarded asbeing in the static regime. T must then be greater than the period ofthe fadings, that is greater than 1/(2*f_(d)), where f_(d) representsthe Doppler frequency, given by the expression f_(d)=f*v/c; f representsthe carrier frequency of the signal and c the speed of light. Todimension T, the variations of f from one channel to another of one andthe same system are disregarded. By way of example, the value of v isfixed at 10 km/h.

During step 342, the maximum and the minimum of the power values P_(C)iover the time interval T are calculated. These two values can also becalculated using respectively the N largest values (in place of themaximum) or the N smallest values (in place of the minimum). Thedeviation between these two extreme values will make it possible toupdate the regime to be considered.

If the deviation is less than a threshold S₁, this signifies that thepower P_(C)i has varied little over the time interval T and that thepropagation is of the static type in the useful channel. A substep 343is then performed which consists in assigning a value corresponding tothis type of propagation to the predetermined mean power level P_(C)o.If conversely the difference is greater than the threshold S₁, then thissignifies that the power P_(C)i has varied significantly over the timeinterval T and hence that the propagation is of the dynamic type. A step344 is then performed which consists in assigning a value correspondingto this type of propagation to the predetermined power level P_(C)o.

In one example, in step 343 the value Pmin+7 dB is given to Po, that isthe predetermined mean power level is 7 dB above the value Pmin of thenoise floor of the converter 16. Correspondingly, the value Pmin+4 db isgiven to the value P_(C)min, this signifying that the acceptable minimumlevel for the power for the useful channel is situated 4 dB above thevalue Pmin of the noise floor of the converter 16. Again likewise, thevalue Pmin+10 dB is given to the value P_(C)max, this signifying thatthe acceptable maximum value of the power in the useful channel issituated 10 dB above the noise floor Pmin of the converter 16. In step344, the values Pmin+15 dB, Pmin+12 dB, and Pmin+18 db are given to thevalues P_(O), P_(C)min and P_(C)max respectively.

Represented in FIG. 6 is a flow chart showing substeps of step 35 ofdetermining a “fading hole” of the signal in the useful channel.

During steps 351 and 352, partly merged with step 341 of FIG. 5, twoconsecutive values P_(C)i and P_(C)i+1 of the power in the usefulchannel are produced. These two values are instantaneous values of thepower in the useful channel. In a step 353, the differenceP_(C)i+1−P_(C)i between these two consecutive values is compared with asecond determined threshold S2 which is greater than the threshold S1mentioned hereinabove in conjunction with FIG. 5. If this difference isless than the threshold S2 then this signifies that the signal in theuseful channel is not in a “fading hole”. This is why in a step 354, thesecond value P_(C)i+1 is taken into account in the calculation of themean values P₁n to P₅n of the power in the useful channel. In theconverse case, represented symbolically by substep 355, this signifiesthat the signal in the useful channel is in a “fading hole”. This is whythe second value P_(C)i+1 is not taken into account in the calculation.It is quite simply ignored.

1. A method of analog/digital conversion of a radiocommunication signalwhose frequency band contains at least one useful channel, using ananalog/digital converter preceded by a variable-gain amplifier whichreceives said signal and which has a gain of a given value, the methodcomprising steps for controlling the value of the gain of the amplifierin such a way that power in the frequency band of the signal is lessthan a limit saturation value of the analog/digital converter and thatpower in the useful channel is substantially equal to a desired meanpower level in the useful channel having a first predetermined value inthe case of static propagation in the useful channel or a secondpredetermined value, different from said first predetermined value, inthe case of dynamic propagation in the useful channel.
 2. The method ofclaim 1 comprising the steps of: comparing values of the power in thefrequency band of the converted signal with the limit saturation valueof the analog/digital converter; and if a value of the power in thefrequency band of the signal is greater than said limit saturation valueof the analog/digital converter minus a predetermined margin, decreasingthe gain of the amplifier in such a way that the power in the frequencyband of the signal is at most substantially equal to a desired meanpower level in the band of the signal.
 3. The method of claim 2, whereinthe values of the power in the frequency band of the converted signaland/or the values in the useful channel which are taken into account inan initialization phase are values of mean power calculated over a timewindow having a first determined width, while the values of the power inthe frequency band of the converted signal and/or the values of thepower in the useful channel which are taken into account in a holdingphase are values of mean power calculated over a time window having asecond determined width, greater than said first determined width. 4.The method of claim 2, wherein the values of the power in the frequencyband of the converted signal and/or the values of the power in theuseful channel which are taken into account are calculated on the basisof measurements of an instantaneous power after a first determined laghas elapsed after bringing into service or modifying a parameter of ananalog part upstream of the analog/digital converter.
 5. The method ofclaim 2, wherein the values of the power in the frequency band of theconverted signal and/or the values of the power in the useful channelwhich are taken into account are calculated on the basis of measurementsof an instantaneous power after a second determined lag has elapsedafter the modification gain of the amplifier.
 6. The method of claim 2,wherein the values of the power in the frequency band of the convertedsignal and/or the values of the power in the useful channel which aretaken into account are converted into values in decibels independent ofthe gain of the amplifier by means of a predetermined conversion table.7. The method of claim 1 comprising a step of determining the type ofpropagation, static or dynamic, in the useful channel.
 8. The method ofclaim 7 wherein the type of propagation in the useful channel isdetermined as a function of the history of the values of the power inthe useful channel which are obtained in the absence of saturation ofthe analog/digital converter.
 9. The method of claim 1 comprising thesteps of: comparing values of the power in the useful channel with saiddetermined mean power level; if a value of the power in the usefulchannel is outside a determined interval around said determined meanpower level, modifying the gain of the amplifier so that the value ofthe power in the useful channel is inside said determined interval. 10.The method of claim 9, wherein the gain of the amplifier is controlledin such a way that the power in the useful channel is substantiallyequal to said determined mean power level.
 11. The method of claim 9,wherein the gain of the amplifier is increased, at most, only by a valuesuch that the power in the frequency band of the converted signalremains less than the limit saturation value of the analog/digitalconverter minus said determined margin.
 12. The method of claim 9,further comprising a step of determining whether the signal in theuseful channel is situated in a fading hole, the gain of the amplifierbeing modified, if appropriate, only if the signal in the useful channelis not situated in a fading hole.
 13. A device for analog/digitalconversion of a radiocommunication signal whose frequency band containsat least one useful channel, comprising an analog/digital converterpreceded by a variable-gain amplifier which receives said signal andwhich has a gain of a given value, and means for controlling the valueof the gain of the amplifier in such a way that power in the frequencyband of the converted signal is less than a limit saturation value ofthe analog/digital converter and that power in the useful channel issubstantially equal to a desired mean power level having a firstpredetermined value in the case of static propagation in the usefulchannel or a second predetermine value, different from said firstpredetermined value, in the case of dynamic propagation in the usefulchannel.
 14. The device of claim 13 comprising: means for comparingvalues of the power in the frequency band of the converted signal withthe limit saturation value of the analog/digital converter; and meansfor decreasing the gain of the amplifier in such a way that the power inthe frequency band of the converted signal is at most substantiallyequal to said limit value minus a determined margin, if a value of thepower in the frequency band of the converted signal is greater than saidlimit saturation value of the analog/digital converter minus saiddetermined margin.
 15. The device of claim 13 further comprising meansfor determining the type of propagation, static or dynamic, in theuseful channel as a function of the history of the values of the powerin the useful channel which are obtained in the absence of saturation ofthe analog/digital converter.
 16. The device of claim 13, furthercomprising: means for comparing values of the power in the usefulchannel with said predetermined mean power level; and means formodifying the gain of the amplifier so that the value of the power inthe useful channel is inside a determined interval around, saiddetermined mean power level, if a value of the power in the usefulchannel is outside said determined interval.
 17. The device of claim 16,further comprising means for determining whether the signal in theuseful channel is situated in a fading hole, the gain of the amplifierbeing modified, if appropriate, only if the signal in the useful channelis not situated in a fading hole.
 18. The device of claim 13, furthercomprising a first unit for measuring power delivering first values ofthe power in the frequency band of the converted signal which are valuesof mean power calculated over a time window having a first determinedwidth, and second values of the power in the frequency band of theconverted signal which are values of mean power calculated over a timewindow having a second determined width, greater than said firstdetermined width.
 19. The device of claim 18, wherein the first and/orthe second unit for measuring power comprise: a) means for producing aseries of successive samples of the signal; b) means for producing aseries of successive instantaneous power values of the signal, each ofthese values being obtained from the value of a respective sample of theseries of successive samples of the signal; c) means for producing Nseries of successive values of the mean power of the signal overrespectively N time windows of respective increasing widths, where N isan integer such that N≧2, from the values of the series of successivevalues of the instantaneous power of the signal.
 20. The device of claim18 further comprising a management unit connected to the output of thefirst and/or of the second unit for measuring power so as to receiverespectively the values of the power in the frequency band of theconverted signal and/or respectively the values of the power in theuseful channel, and means for compensating for any difference of delayin the transmission of these respective values which is due to adifference of the paths taken, so as to deliver at the input of saidmanagement unit values of the power in the converted band and values ofthe power in the useful channel referring to identical samples of theconverted signal of the converted signal.
 21. The device of claim 13,further comprising a second unit for measuring power delivering firstvalues of the power in the useful channel which are values of mean powercalculated over a time window having a first determined width, andsecond values of the power in the useful channel which are values ofmean power calculated over a time window having a second determinedwidth, greater than said first determined width.